1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly, to a method of removing a silicon nitride film formed on the bottom of a contact hole and the like in fabrication of a semiconductor device.
2. Description of the Related Art
With the miniaturization in fabrication processes of semiconductor devices, the miniaturization is also under way in contact holes, via holes and the like formed through interlayer insulating films in semiconductor devices. Since interlayer insulating films cannot be reduced in thickness in concert with the progress of miniaturization in design rules, the aspect ratios of holes such as contact holes necessarily become larger. In addition, due to a requirement for reducing variations in alignment of contact holes with underlying wires and the like, associated with the miniaturization in the fabricating process, a self-aligned contact (SAC) process has drawn more and more attention because this process can eliminate a design margin for the alignment on a photomask.
While there are several SAC processes, a typical one involves forming a gate electrode and a gate wire, an offset oxide film disposed on the top faces of them, and side walls (oxide films) disposed on side faces of the gate electrode and gate wire, conformally forming a thin SiN (silicon nitride) film over the entire surface as an etching stopper film, subsequently forming an interlayer insulating film (oxide film), and selectively removing the interlayer insulating film at and near positions at which contact holes are to be formed through a photolithographic step. In this event, since the final positions of the contact holes are determined by the side walls and offset oxide film, this can be said a sort of self-aligned process. Finally, the SiN film is removed from the bottoms of the contact holes which are then filled with contact plugs.
Further, in the SAC process, an attempt has been made to use an SiN film instead of the offset oxide film formed on the gate electrode and gate wire, and to use an SiN film as well for the side walls.
Depending on a process which follows the formation of contact holes, the interlayer insulating film, which is an oxide film, can be damaged. For protecting the interlayer insulating film from such a damage, a thin SiN film may be formed on a side wall (inner wall) of a formed contact hole, for example, in thickness of 10 to 20 nm. In this case, the SiN film on the bottom of the contact hole must be removed after the SiN film is formed on the side wall of the contact hole. Particularly, the formation of such SiN film is deemed as essential when a relatively xe2x80x9csoftxe2x80x9d oxide such as BPSG (borophosphosilicate glass) is used as the interlayer insulating film.
FIG. 1 is a schematic cross-sectional view illustrating a contact hole before an SiN film is removed after the contact hole was formed. A pair of wiring patterns 12 or electrode patterns made of WSi (tungsten silicide) are formed on substrate 11 made of silicon or the like, and similarly patterned offset SiN films 13 are formed on wiring patterns 12. Further, side walls 14 similarly made of SiN are provided on side faces of wiring patterns 12 and offset SiN films 13. Then, interlayer insulating film 15 made of silicon oxide is formed over the entire surface of substrate 11 including wiring patterns 12, offset SiN films 13 and side walls 14. Interlayer insulating film 15 is formed with a contact hole 16 by an SAC process. Contact hole 16 extends through interlayer insulating film 15 to the surface of substrate 11 in a region sandwiched by the pair of wiring patterns 12.
Thin SiN film 17 is formed on the bottom and inner side face (side wall) of contact hole 16. Here, the bottom of contact hole 16 refers to a portion of the contact hole which is in contact with the surface of substrate 11. This SiN film 17 is provided as a film for protecting interlayer insulating film 15 from a wet etching and the like in subsequent processes. While SiN film 17 is also formed on the top face of interlayer insulating film 15 depending on its deposition process, the SiN film overlying interlayer insulating film 15 may be removed as required by a subsequent CMP (chemical mechanical polishing) process or the like. It should be noted that contact hole 16 formed by the SAC process is generally set such that its diameter on the top face of interlayer insulating film 15 is larger than that on the bottom of contact hole 16. Therefore, the diameter of contact hole 16 on the bottom thereof is determined by side walls 14 in a self-aligned manner, and shoulder 18 is formed within contact hole 16.
When contact hole 16 is used for interlayer connection, SiN film 17 must be removed from the bottom of contact hole 16, as described above, before contact hole 16 is filled with a wiring material or a wiring plug. In this event, since the SiN film on the side wall of contact hole 16 must be left, anisotropic etching is used. Such anisotropic etching may be dry etching such as plasma etching.
When the SiN film on the bottom of contact hole 16 is removed by plasma etching, a gas system conventionally used for this purpose is a gas system of CHF3/Ar/O2, a gas system of CH2F2/Ar/O2, and the like. When the former gas system is used, an etching reaction is expressed by:
Si3N4+4CHF3xe2x86x923SiF4↑+4HCN↑
In this example, a product having a relatively high vapor pressure, such as SiF4, HCN or the like is formed to promote the etching reaction. Likewise, with the latter gas system a product having a relatively high vapor pressure is produced such as SiF4, HCN or the like.
However, the dry etching which uses the aforementioned conventional gas system has a problem in that the SIN film on the side face of the contact hole, as well as the SIN film on the bottom of the contact hole is inevitably etched to some degree. Since etching is less advanced as the aspect ratio is larger, in other words, since an upper portion of a contact hole, i.e., a region near the entrance of the contact hole is etched in advance, the etching rate at the shoulder 18 within the contact hole is higher than the etching rate on the bottom of the contact hole. In some cases, as illustrated in FIG. 2, an SiN film on side wall 14 has been etched away before the SIN film on the bottom of contact hole 16 is completely removed, causing wiring patterns 13 made of WSi to expose to contact hole 16. If contact hole 16 is embedded with a contact plug metal with exposed wiring patterns 13, the contact plug will be short-circuited with wiring patterns 13.
It is therefore an object of the present invention to provide a method of removing a silicon nitride (SiN) film which is capable of reliably removing an SiN film on the bottom of a contact hole without removing an SiN film formed on a side wall of the contact hole, even if the contact hole has a large aspect ratio.
The inventor diligently repeated investigations for achieving the above object, and as a result found that a silicon nitride film on the bottom of a hole such as a contact hole alone can be selectively removed by using a process gas which comprises a first fluorine compound including a carbon atom-carbon atom bond, and a second fluorine compound including at least one hydrogen atom and a single carbon atom in one molecule, thereby completing the present invention. Assume in the present invention that a double bond Cxe2x95x90C and a triple bond Cxe2x89xa1C also fall under the carbon atom-carbon atom bond, in addition to the single bond Cxe2x80x94C. In the present invention, preferably used as the first fluorine compound may be, by way of example, octafluorocyclobutane (C4F8), hexafluorobutadiene (C4F6), octafluorocyclopentene (C5F8) and the like. On the other hand, preferably used as the second fluorine compound may be, by way of example, monofluoromethane (CH3F), difluoromethane (CH2F2), trifluoromethane (CHF3) and the like.
Specifically, a method of removing a silicon nitride film according to the present invention has an application in removal of a silicon nitride film formed on a surface of a material. The method includes the steps of supplying a process gas which comprises a first fluorine compound having fluorine atoms and at least two carbon atoms in one molecule, and a second fluorine compound having at least one fluorine atom, at least one hydrogen atom and a single carbon atom in one molecule, and performing dry etching using the process gas to remove the silicon nitride film.
In the following description, for facilitating the understanding the invention by comparison in terms of chemical structures, the first fluorine compound, i.e., a fluorine compound including a carbon atom-carbon atom bond may be called the xe2x80x9chigher-order fluorocarbonxe2x80x9d and the second fluorine compound, i.e., a fluorine compound including at least one hydrogen atom and a single carbon atom in one molecule, the xe2x80x9clower-order fluorocarbon.xe2x80x9d
Conventionally, higher-order fluorocarbons such as octafluorocyclobutane (C4F8), hexafluorobutadiene (C4F6), octafluorocyclopentene (C5F8) and the like have been used for etching silicon oxide films, particularly for removing a silicon oxide film formed on the bottom of a hole having a large aspect ratio, such as a contact hole. However, due to a low vapor pressure of a product obtained by a reaction with silicon nitride, and high probability of producing deposition, the higher-order fluorocarbons are deemed as substantially failing to provide etching performance for a silicon nitride film, so that the higher-order fluorocarbons have not been used for etching a silicon nitride film in any examples.
However, the inventor has found that a silicon nitride film alone can be selectively removed on the bottom of a hole such as a contact hole by using such a higher-order fluorocarbon in combination with a lower-order fluorocarbon such as fluoromethane (CH3F), difluoromethane (CH2F2), trifluoromethane (CHF3) or the like. The lower-order fluorocarbon is conventionally used for etching silicon nitride films. Specifically, the inventor has found that a silicon nitride film formed on the bottom of a contact hole or the like can be completely removed without etching a silicon nitride film formed on a side wall of the contact hole or the like by supplying a process gas including a higher-order fluorocarbon and a lower-order fluorocarbon, and performing dry etching using this process gas.
As a mechanism of the foregoing phenomenon, the inventor assumes a process as follows: As a higher-order fluorocarbon is decomposed within a plasma, CxFy radicals, derived from the higher-order fluorocarbon, increase in the vapor phase. The CxFy radicals, however, can be introduced a shallow distance into a hole, but not deep into the hole due to the adsorption characteristic thereof. On the other hand, radicals derived from a lower-order fluorocarbon is introduced deep into the hole to etch an SiN film. Thus, the bottom of the contact hole is etched by the radicals derived from the lower-order fluorocarbon, while a side wall and a shoulder of the contact hole are protected by an inverse micro-loading effect of the CxFy radicals derived from the higher-order fluorocarbon. In this manner, the SiN film alone is selectively etched on the bottom of the hole. The inverse micro-loading effect, herein referred to, means that a location shallow in a hole is not etched but a location deep in the hole is etched.
The inventor further investigated a preferred ratio of the higher-order fluorocarbons (first fluorine compounds) to the lower-order fluorocarbons (second fluorine compounds) in a supplied process gas, and eventually found that the following relationship is preferably satisfied:
1xe2x89xa6R1/R2xe2x89xa64 
where R1 is the sum of m1xc3x97nC-C calculated for the respective first fluorine compounds; R2 is the sum of m2xc3x97nC-H calculated for the respective second fluorine compounds; nC-C is the number of carbon atom-carbon atom bonds included in one molecule of each first fluorine compound; m1 is a mole fraction of each first fluorine compound in the supplied process gas; nC-H is the number of carbon atom-hydrogen atom bonds included in one molecule of each second fluorine compound; and m2 is a mole fraction of each second fluorine compound in the supplied process gas.
The method of removing a silicon nitride film according to the present invention is particularly suitable for removing a silicon nitride film formed on the bottom of a hole such as a contact hole formed in fabrication of a semiconductor device. Such a hole may be a hole, for example, having an aspect ratio in a range of three to ten.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.